The present invention relates to electrical, high-frequency filters using the frequency selectivity of the generation and pick-up of acoustic surface waves on a suitable piezoelectric material to obtain band pass filtering; such filters are also called surface acoustic wave filters for short.
A typical filter of the type referred to above includes a flat piezoelectric wafer as substrate which carries at least two sets of interdigitized comb-like finger electrodes, one set or pair of combs acting as transmitter for acoustic surface waves, the other set or pair of combs acts as pick-up or receiving transducer. A voltage applied between adjacent fingers of the one pair of combs deforms the crystal elastically resulting in an elaastic wave which propagates across the surface of the piezoelectric body. An elastic deformation polarizes the affected surface of the crystal and sets up a voltage between the electrode fingers of the other pair of combs provided these fingers are spaced in accordance with the polarization pattern that results from the particular surface wave.
It is inherent in such a device that either transducer is not inherently a transmitter or a receiver. Rather, the interaction between the electrode fingers on the piezoelectric substrate is strictly reversible. An elastic deformation in the substrate produced an electric voltage potential which can be detected (and current can be drawn) by a conductive electrode right at the deformed substrate surface; a voltage potential of such an electrode relative to the substrate body or relative to a different surface portion produces an elastic deformation of the substrate. Thus, the voltage between a pair of combs resulting from an arriving surface wave, causes this transducer to launch surface waves on its own. Consequently, an arriving signal will be retransmitted. Moreover, the terms transmitting transducer and receiving transducer have meaning only with regard to their use in an electric circuit; an electrical signal is applied to one and an electrical signal is extracted from the other. Such connections and use do not eliminate from the transmitting transducer its capability of responding to any incoming surface wave nor is the receiving transducer disabled for transmitting surface waves.
As a consequence of the afore-mentioned phenomenon, such a two transducer device exhibits so-called triple transit echos. Specifically, as the transmitting transducer launches a surface wave, the receiving transducer picks it up and generates a corresponding output voltage. However, the receiving tranducer as so stimulated acts also as transmitter and returns an acoustic signal to the transmitter proper. A portion of that signal is received by the transmitting transducer and retransmitted again (second echo), and another acoustic signal returns to the pick-up and receiving transducer, having traversed the distance between the transducer three times as compared with the single transit that occurred on initial stimulation by the transmitter.
If that triple transit echo were very low, it could be neglected. However, it may be only 12 db down from the proper signal level. The triple transit echo has, of course, at least substantially the same informaton content as the original signal as received by the receiving transducer, but that echo arrives one full round trip between the transducers later. As that delay may involve a few microseconds it is per se not negligible and may appear, for example, as a "ghost" if the filter is used in an i.f. channel in a television set. In addition, these triple transit echoes distort the band pass frequency response of such a device in that the response exhibits a ripple with an undulation of about 4 db.
Strictly speaking, the echo producing process continues resulting in quintuple, septuple, etc. transit echoes. However, these echo signals produce relatively insignificant disturbences. Moreover, upon reducing triple transit echoes one can expect these other echoes to be correspondingly reduced; i.e. they do not present a separate problem. On the other hand, the triple transit echo is just one example for echo signals. Surface waves or any other elastic distortion in the substrate will cause e.g. the transmitting transducer to act as receiver and retransmitter. Thus, unless other echoes and other types of reflections are inhibited otherwise, they too will produce echo signals in the receiver proper.
Various attempts have been made to impede the retransmission of signals by a receiving transducer. However, these attempts are related primarily to low terminating resistances which are insufficient and tend to introduce noise.